1. Field of the Invention
This invention relates to an apparatus for transporting a web of a flexible material between a first and a second position. In particular, and in a preferred embodiment, the invention provides an apparatus, known as a "vacuum conveyor", for transporting a web, preferably a lead strip of a paper web from the dryer of a paper-making machine to the first nip of a calender of the machine or to a winding machine or between any other sections of a paper-making or paper-finishing (e.g. coating) machine.
The present invention is intended as an improvement over the operation of the apparatuses described in U.S. Pat. Nos. 3,355,349 and 4,022,366. All features disclosed in these patents are incorporated into the present patent application by reference.
2. Discussion of Background Information
U.S. Pat. No. 3,355,349 issued on Nov. 28, 1967 describes an apparatus for transporting a lead strip of paper web from the dryer of a paper-making machine and feeding into the first roll nip of the calender of said machine. The apparatus comprises an elongated frame, an air-pervious (air permeable) endless belt movably mounted on the frame and having one run, e.g. its upper run travelling from the dryer to the first roll nip of the calender. The belt is arranged to receive the lead strip from the dryer. A vacuum chest having a perforated top extends longitudinally of and below the upper run of the belt. There are device for evacuating air from the chest so as to apply the lead strip to cling to the belt. Air blast device beneath, and extending transversely of, the upper run of the belt are positioned at the discharge end. These air blast device direct a blast of air upwardly through the belt to lift the lead strip from the belt and extend the lead strip towards the first roll nip of the calender. Thus, the apparatus described in the above United States patent is provided with device for ensuring that the lead strip is temporarily attached to the belt and with device to ensure that the lead strip can leave the belt at the appropriate time.
U.S. Pat. No. 4,022,366 issued on May 10, 1977 provides an improvement to the apparatus of the above U.S. Pat. No. 3,355,349. Said improvement comprises an opening where the web or lead strip leaves the belt and an air jet device able to direct air through the opening in a direction contrary to the direction of movement of the belt and with a strength sufficient both to overcome any gripping effect of air entrained within the air-pervious belt and to lift the web of flexible material.
In a preferred embodiment, the above apparatus has a discharge shoe or "nose shoe" disposed beyond the second pulley and spaced slightly from the second pulley to define an opening between the shoe and the second pulley. Provided is an inlet into the shoe to receive a supply of air, a plurality of outlets disposed in the nose shoe adjacent the second pulley whereby an air jet may be directed through the opening and against the web in a direction contrary to the rotation of the second pulley.
The above apparatus has been found to be effective with paper of the heavier basis weights. The system is able to project such paper forward and, by varying the air flow, may control the angle of trajectory in relation to the belt face. However, lightweight tissues still give problems. Lightweigt tissue has been found to fall over the end of the shoe out of control. Accordingly, in a further improvement, the apparatus of U.S. Pat. No. 4,022,366 includes a second air jet device, downstream from the first air jet (that is the device able to direct air through the opening); the second air jet device is able to direct air downstream to act as a platform for the web. The second air jet device is preferably provided with holes that are small in diameter to give a velocity flow with minimum air volume. The tissue web will not fall through this air stream until the stream is too weak to provide support.
The improved apparatus works well in most circumstances but is has been found that as the velocity of the paper-making or paper-finishing machines is increased up to more than 2000 meters per minute, it becomes more and more difficult to properly project the web or lead strip from the downstream end of the apparatus to the following structure or to the following section of the machine. The reason for that problem is that the web or lead strip tends to twist so that it cannot enter in a straight line into the following structure of section, e.g. into the first nip of a calender. The present invention seeks to avoid this disadvantage and to provide a further improvement to the apparatus described above.